1. Field of the Invention
The present invention relates to a nitride based semiconductor light emitting diode (LED) that can be driven at a low driving voltage by improving the current spreading efficiency.
2. Description of the Related Art
Generally, nitride based semiconductors are group III-V semiconductor crystals having an empirical formula of AlXInYGa1-X-YN (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1). The nitride based semiconductors are widely used as LEDs that emit short wavelength light (ranging from ultraviolet light to green light), especially a blue light.
The nitride based semiconductor LEDs are formed on an insulating substrate, such as a sapphire substrate or SiC substrate, which meets lattice matching condition for crystal growth. Two electrodes connected to a p-type nitride semiconductor layer and an n-type nitride semiconductor layer have a planar structure. That is, the two electrodes are arranged almost horizontally on an emission structure.
The nitride based semiconductor LEDs having the planar structure must have high brightness when they are to be used as a lighting source. In order to obtain the high brightness, large-sized nitride based semiconductor LEDs operating at a high current have been manufactured.
However, the nitride based semiconductor LEDs has such a vertical structure that two electrodes are respectively arranged on the top and bottom surfaces of the emission structure. Compared with the nitride based semiconductor LEDs having the vertical structure, the large-sized nitride based semiconductor LEDs having the planar structure have non-uniform current flow in an entire emission region. Thus, the effective area used for light emission is not so wide that the emission efficiency is low.
In addition, when the high current is applied to the large-sized nitride based semiconductor LED having the planar structure, the applied current is converted into heat, so that the temperature of the LED increases. Consequently, the driving voltage and characteristic of the LED is degraded.
Hereinafter, the problems of the large-sized nitride based semiconductor LED having the planar structure according to the related art will be described with reference to FIG. 1.
FIG. 1 is a plan view illustrating the arrangement of electrodes and an active region in a nitride based semiconductor LED according to the related art.
Referring to FIG. 1, the top surface of the LED includes an n-type nitride semiconductor layer 120, an active layer, and a p-type nitride semiconductor layer (not shown), which are sequentially formed on a substrate.
A p-electrode 160 and an n-electrode 150 are formed on the top surface of the LED. The p-electrode 160 and the n-electrode 150 are connected to the p-type nitride semiconductor layer (in some cases, a reflective electrode 170) and the n-type nitride semiconductor layer 120, respectively.
The n-electrode 150 includes two electrode pads 155 and a plurality of branch electrodes 150′ and 150″ extending from the electrode pads 155. The n-electrode 150 and the p-electrode 160 have a finger structure such that they are meshed with each other. At this point, the p-electrode 160 is divided by the branch electrodes 150′ and 150″ of the n-electrode 150, so that the emission area is divided.
That is, two n-type electrode pads 155 are formed on the n-electrode 150. When a high current is applied, the applied current is dispersed and the current spreading efficiency is improved by arranging the p-electrode 160 and the plurality of branch electrodes 150′ and 150″ extending from the n-electrode 150 in the finger shape. Consequently, failure of the device due to the application of the high current can be prevented.
In the large-sized nitride based semiconductor LED according to the related art, the high current is dispersed by two n-type electrode pads, thereby preventing the degradation in the driving voltage and characteristic. However, because the two n-type electrode pads are concentrated on one side of the LED, there is a limitation in improving the current spreading efficiency and reducing the driving voltage of the LED.
Therefore, there is an increasing demand for a technology that can improve the current spreading efficiency and reduce the driving voltage in the large-sized nitride based semiconductor LEDs.